The present invention concerns thermostatic or thermally controlled fluid mixing valves. In particular, the invention concerns a mixing valve having means for compensating for the failure of the thermstat element or thermally responsive control member.
Thermally controlled or thermostatic mixing valves are well known. Many such fluid mixing valves include a single control knob that can be rotated to vary the temperature of the output fluid between "off", "cold" and "hot" positions. A valve member is controlled by a thermally responsive control member, or thermostat, to maintain the fluid temperature according to the setting of the control knob.
Thermally operated mixing valves are frequently used to control the temperature water through the shower fitting. Other applications include an emergency eye-wash fountain in which tepid water is provided for washing the eyes of a victim of some contamination. In both cases, it is important that the mixing valve properly operate to maintain the pre-set temperature of the flow of liquid from the valve. In many cases, the mixing valve maintains the temperature of the output liquid to within plus or minus 2.degree. F. of the pre-set temperature.
In an application of mixing valves in which the output fluid is in contact with a user, it is critical that some form of emergency shut-off be provided in the event of a failure of at least the cold fluid source. In the absence of such an emergency mechanism, the user may be scalded by the hot water flowing exclusively through the valve in the absence of cold mixing water, during the lag before the thermostat functions to shut the valve off. In other applications, such as the eye-wash application, it is also important that the mixing valve has some means for maintaining some flow of liquid through the valve in the event of a failure of the valve or thermal control member. In the eye-wash application, it is imperative that at least some cold water flow is maintained, otherwise the victim may suffer serious injury for failure to wash the eye completely.
It has been found that prior art mixing valves do not adequately address the potential problems of failure of the thermally responsive control member or thermostat. Moreover, it has been found that these valves do not have such a failure mode in which fluid is still permitted to flow through the valve, albeit it at a reduced flow rate. Moreover, it is believed that no prior art mixing valve provides a mechanism to address the valve failure modes, that does not also severely impact the flow of fluid through the valve under normal operating conditions.